1. Field of the Invention
The present invention relates generally to webbing, and in particular to woven webbing structures for vehicle occupant restraining used in a vehicle seat belt device.
2. Description of the Related Art
An example of a vehicle seat belt device is a three point seat belt type using a continuous webbing.
In this type of seat belt device, one end of the webbing is fastened to a take-up mechanism and another end of the webbing is fastened to an anchor plate via a through anchor. In order to apply the webbing to a vehicle occupant, a tongue plate provided at an intermediate portion of the webbing between the anchor plate and the through anchor is engaged with a buckle device and the webbing is thereby pulled out from the take-up device.
A type of webbing used in the above-described seat belt device is known as so-called energy absorbing webbing, which has a function of reducing (absorbing) energy that acts on a vehicle occupant at the time of sudden deceleration of a vehicle. This type of webbing is comprised of different kinds of yarns running along a longitudinal direction of the webbing and having different elongation percentages or different extensional rigidities. For example, the warps are formed by cored yarns whose elongation percentage is relatively small or whose extensional rigidity is relatively large, along with side yarns having a large elongation percentage or a small extensional rigidity.
As a result, when a large load acts on the webbing, the cored yarns are broken earlier than the side yarns and absorb the load, thereby reducing energy applied to the vehicle occupant.
In such a type of conventional energy absorbing webbing, it is thus possible to reduce energy acting on the vehicle occupant due to the cored yarns being broken and absorbing the load. However, the cored yarns interwoven within the webbing come out from the surface of the webbing due to friction caused by the webbing sliding on the through anchor or the anchor plate, or bending of the webbing caused by applying the webbing to the vehicle occupant. As a result, the yarns fray, or deterioration in appearance occurs.
In view of the above-described facts, an object of the present invention is to provide a woven webbing structure, in which the risk of (often referred to herein as xe2x80x9cwebbingxe2x80x9d for convenient reference) interwoven cored yarns coming out from a surface of the webbing due to friction caused by the webbing sliding on a through anchor, or bending of the webbing caused by applying the webbing to a vehicle occupant, is reduced. The advantage thereof is to thereby prevent occurrence of frayed yarns or deterioration in appearance. It is intended to achieve these effects without significantly reducing strength or energy absorptivity of the webbing.
A woven webbing structure according to the present invention comprises plural types of warps interwoven along a longitudinal direction of the webbing and having different elongation percentages or extensional rigidities, and wefts interwoven along a transverse direction with respect to the warps. In this woven webbing structure, among the plural kinds of warps, there is at least one kind of warp whose elongation percentage is relatively small or extensional rigidity is relatively large, and which has been subjected to twist processing.
In the woven webbing structure according to the present invention, the warps interwoven along the longitudinal direction of the webbing each include plural kinds of yarns having different elongation percentages or extensional rigidities. Among these warps, a yarn whose elongation percentage is relatively small or extensional rigidity is relatively large has been subjected to twist processing.
The above-described webbing is used in, for example, a vehicle safety restraint belt device. When a large load acts on the webbing, the yarn whose elongation percentage is relatively small or extensional rigidity is relatively large among the warps breaks earlier than other yarns and absorbs the load, thereby reducing energy that would act on the vehicle occupant.
Further, in the woven webbing structure according to the present invention, the warp whose elongation percentage is relatively small or extensional rigidity is relatively large has been subjected to twist processing. Therefore, unevenness is formed on the surface of this warp and friction between this yarn and other yarns (other warps or wefts) increases. The frictional force functions for preventing free movement of the twisted warp due to bending of the webbing, or the like. Accordingly, even if the webbing is bent by friction due to the webbing sliding on a through anchor or applying the webbing to a vehicle occupant, the above-described twisted warp resists coming out from the surface of the webbing.
Particularly, when plural kinds of warps having different thicknesses are subjected to twisting processing (for example, when two kinds of warps having different thicknesses are twisted together), the entire surface of these warps is made uneven still more so as to further increase the frictional force between yarns, which is even more effective.
As described above, in the woven webbing structure according to the present invention, there is reduced possibility that the interwoven yarns will come out from the surface of the webbing due to bending of the webbing, or the like, such as when the webbing is applied to a vehicle occupant. As a result, fraying of yarns and deterioration in appearance is reduced, and is achieved without any significant reduction in the strength and energy absorptivity of the webbing.
Further, according to another aspect of the present invention, in the above-described woven webbing structure, the warp is subjected to single twisting or plying and is set such that a twist coefficient thereof, K, is at least 1200, where the number of twists per meter is T, the denier number of the warp is D, and the twist coefficient defined by
T{square root over ( )}D is K. 
When the twist coefficient K is set as described above, friction between yarns due to surface unevenness of the warp increases in rapid proportion to K, so that the warp resists coming out from the surface of the webbing.